The decoder of the present invention is especially well suited for use in fast applications. Various systems have been devised to control raster output in printing systems. For example, in U.S. Pat. No. 4,300,206 an image generator is described for positioning text and image data of any size or orientation on a page.
In U.S. Pat. No. 4,079,458 a high resolution character generator is described for producing rows of characters to be scanned on a display medium wherein the input data defining characters to be printed in ordered rows of text is sorted to provide specifications for characters on each successive scan line. The data relating to the characters to be printed on lines are sorted in such a way that all of the characters which start on the same scanning line are grouped together. The data of the characters which start on successive scanning lines are stored in an input memory, and the data relating to partially processed characters are stored in an active memory. Pixel pattern data for a number of characters are stored at separately accessible address locations in a front memory and are retrieved as a function of the data from the input memory and the activity memory during the successive scanning lines. The retrieved data are placed in an output memory and are then directed in the form of a serial pixel data flow to an output device such as an xerographic printer. To increase the processing speed, two output memories are used such that while data is being read out of the first output memory, the second output memory can be loaded. Notwithstanding the use of two output memories, processing speed is not sufficiently fast.
Various other page printing schemes have been proposed for converting high level encoded page layout information into a printed page, see International Application No. 83/03487 (U.S. Ser. No. 363,761 now abandoned and U.K Application No. GB 2 119 982 A. Also of interest is U.S. Pat. No. 4,143,950 which discloses apparatus for assembling binary data bits in parallel by groups in variable, selected locations in a shift for subsequent serial readout.
Because many of the prior art systems utilize word displacement in conformity with a relevant address as well as the nature of output memory devices, the decoding schemes are not conductive to high speed decoding. Accordingly, it is an object of the present invention to overcome the limitations of prior art decoders, and, in particular, to overcome the limitations in speed in decoding compiled page information for use by a raster output scanner.